The present invention relates to transformer windings and, more particularly, to insulated transformer windings having insulated breakouts and methods for forming the same.
Certain safety regulations relating to insulation between transformer windings require that the transformer be designed such that a stipulated winding creepage distance is allowed without contact between respective windings and such that a stipulated clearance between respective windings is provided. The creepage distance is the shortest distance between adjacent conductors following a surface without going through insulation (rather, the distance is measured as going around and/or along insulation). Additionally, regulations may require certain minimum air distances and distances through insulation between windings. It is desirable to meet such regulations while nonetheless reducing the cost and size of the transformer.
One method that has been used to meet the foregoing regulations is to provide substantial margins as illustrated by the transformer 10 as shown in FIG. 5, which may be referred to as a xe2x80x9cmargin coil designxe2x80x9d. The transformer 10 has a core 12 with a core center leg 12A and opposed end legs 12B. The transformer 10 also includes a first, foil winding 30 that is wound about the center leg 12A, and a second, wire winding 20 that is wound about the first winding 30. Alternatively, the second winding 20 may be a foil winding also or the positions of the windings 20 and 30 may be reversed. The core 12 may include an insulating cover layer to prevent direct electrical contact with either of the windings 20, 30. Insulation layers 14, 16, 18 (which may be reinforced) are inserted between the center leg 12A and the first winding 30, between the first winding 30 and the second winding, and between the second winding 20 and the end legs 12B of the core 12.
In order to meet the above-mentioned required creepage distance and clearance (RCDC), primary margins M1 and M2 are provided above and below the winding 30 and secondary margins P1 and P2 are provided above and below the winding 20. The required margins may depend on the voltage class of the transformer, the class of the insulation employed and/or other parameters. Typically, the sum of the shortest primary and secondary margins M1, M2, P1, P2 should be greater than or equal to the RCDC. That is (regarding the core as electrically conductive), the margins and the RCDC should be related as follows:
M1+P1xe2x89xa7RCDC
M2+P2xe2x89xa7RCDC
M1+P2xe2x89xa7RCDC
M2+P1xe2x89xa7RCDC
The combined width of the winding 20 and the margins P1, P2 and the combined width of the winding 30 and the margins M1, M2 are each limited by the length L of the core center leg 12A. The widths of the margins M1, M2, P1, P2 may be substantial as compared to the widths of the windings 20, 30. Hence, a large portion of the available winding width may be consumed by the margins M1, M2, P1, P2, thereby necessitating the provision of a larger core and, accordingly, a larger transformer.
In order to provide better utilization of the available winding space, a transformer as described above may be formed without margins, i.e., with the widths of the windings being of nearly the same dimension as the length of the core center leg 12A. An exemplary margin free coil transformer 10xe2x80x2, which may be referred to as a xe2x80x9cmargin free coil designxe2x80x9d, is shown in FIGS. 6 and 7. The transformer 10xe2x80x2 has windings 20xe2x80x2, 30xe2x80x2, reinforced insulating layers 14xe2x80x2, 16xe2x80x2, 18xe2x80x2, and a core 12xe2x80x2 having a center leg 12Axe2x80x2. Each winding 20xe2x80x2, 30xe2x80x2 has a breakout on each end thereof. The breakouts 34xe2x80x2 of the foil winding 30xe2x80x2 are shown in cross-section in FIG. 7.
Notably, means must be provided in the margin free coil transformer 10xe2x80x2 to address the creepage distance and clearance regulations discussed above. One method of solving this problem is to insulate the first (foil) winding 30xe2x80x2 and its breakouts in their entireties such that the requirements for creepage distance and clearance, as well as distance through insulation, are met by the insulation about the first winding 30xe2x80x2 alone.
For example, a winding foil strip 40 as shown in FIG. 8A may be provided. The strip 40 has a width that is approximately the same as the length of the center leg 12Axe2x80x2. The strip 40 is covered with an insulator 40A and then folded once to create a breakout 42 of the same width as the strip 40, as shown in FIG. 8B. However, in many transformers the width of the center leg 12Axe2x80x2 is substantially less than its length and the breakout should be close to the width of the core. For example, in ferrite EE-cores the length to width ratio of the center leg is typically approximately two. To achieve the appropriate breakout width, the breakout 42 is folded again to form a narrow breakout 44 as shown in FIG. 8C. The breakout 44 corresponds to one of the breakouts 34xe2x80x2 (see FIG. 7), for example. Notably, this method of folding creates substantial increases in thickness in certain areas as a result of the stacking of four layers of foil, as well as the insulation, on each layer. Additionally, the insulation may be damaged by the folding steps. If holes are formed in the insulation, the transformer may no longer meet the creepage distance, clearance and distance through insulation requirements. The existence of small holes in the insulation may be hard to detect.
According to a further prior art method, a triple insulated wire which is approved by safety agencies for use where reinforced insulation is required may be used for the winding 20xe2x80x2 without additional insulation. The wire in the winding 20xe2x80x2 itself provides the required insulation and there are therefore no requirements on the insulation of the winding 30xe2x80x2 other than functional requirements. This method suffers from several drawbacks in practice.
As an alternative to using a folded foil winding, the winding 30xe2x80x2 may be formed using an insulated winding foil strip 50 and a joined breakout 52 as shown in FIGS. 9A and 9B. The breakout 52 corresponds to one of the breakouts 34xe2x80x2 (see FIG. 7). The breakout 52 and the strip 50 are each covered with an insulator 50A, 52A except on end portions 50B, 52B. The end portions 50B, 52B are exposed to allow electrical contact between the strip 50 and the breakout 52 over most of the width of the strip 50. According to some prior art methods, one or more supplemental insulation members may be provided covering the exposed portions of the winding foil strip and the breakout. However, such constructions may not in fact provide a true margin free coil design while still meeting applicable safety requirements and, accordingly, margins are still required.
The invention is directed to winding assemblies for use in transformers and methods for forming the same. The winding assemblies include one or more foil strips and insulation covers arranged to provide electrically insulated winding and breakout or breakout tap portions. The winding assemblies may be constructed so as to meet the aforementioned creepage distance and other requirements.
According to method embodiments of the invention for forming an insulated winding assembly for an electrical transformer, an integral foil strip having a lengthwise axis is provided. The foil strip includes a winding portion and a breakout portion extending from the winding portion along the lengthwise axis. The breakout portion is folded about the winding portion to form a fold between the breakout portion and the winding portion. Thereafter, an insulation cover is secured to the foil strip.
The step of securing an insulation cover to the foil strip may include securing a first insulation cover to the winding portion and securing a second insulation cover to the breakout portion such that a contact portion of the foil strip adjacent the fold remains exposed. The method further includes securing a supplemental insulation cover over the contact portion. The second insulation cover may be overlapped over the winding portion by at least a prescribed minimum creepage distance.
According to further method embodiments of the invention for forming an insulated winding assembly for an electrical transformer, a foil winding strip having a lengthwise axis and a first insulation cover covering a portion of the winding strip are provided. An exposed portion of the winding strip extends beyond the first insulation cover along the lengthwise axis of the winding strip. A foil breakout strip having a lengthwise axis and a second insulation cover covering a portion of the breakout strip is also provided. An exposed portion of the breakout strip extends beyond the second insulation cover along the lengthwise axis of the breakout strip. The breakout strip is placed on the winding strip such that the breakout strip exposed portion engages the winding strip exposed portion and the second insulation cover overlaps the winding strip.
The second insulation cover may overlap the winding strip by at least a prescribed minimum creepage distance. A supplemental insulation cover may be secured over the breakout strip exposed portion.
According to other method embodiments of the invention for forming an insulated winding assembly for an electrical transformer, a foil winding portion and a foil breakout portion adjoining the winding portion are provided. The breakout portion overlaps and extends at an angle with respect to the winding portion. A supplemental insulation cover is wrapped about and between the winding portion and the breakout portion such that the supplemental insulation cover includes a first panel covering the breakout portion, a second panel covering a rear surface of the winding portion, and a third panel disposed between the winding portion and the breakout portion.
According to other method embodiments of the invention for forming an insulated winding assembly for an electrical transformer, a foil winding strip and a foil breakout strip are provided. The foil winding strip has a lengthwise axis and a first insulation cover covering a portion of the winding strip. An exposed portion of the winding strip extends beyond the first insulation cover along the lengthwise axis of the winding strip. The foil breakout strip has a lengthwise axis and a second insulation cover covering a portion of the breakout strip. An exposed portion of the breakout strip extends beyond the second insulation cover along the lengthwise axis of the breakout strip. The breakout strip is placed on the winding strip such that the breakout strip exposed portion engages the winding strip exposed portion and the breakout strip and the winding strip form an inner corner therebetween. A bellows cover is placed over the breakout strip and the winding strip such that a first leg portion of the bellows cover overlaps the first insulation cover, a second leg portion of the bellows cover overlaps the second insulation cover, and a bellows section of the bellows cover extends across the inner corner and joins the first and second leg portions. The method may further include placing an L-shaped cover over the breakout strip and the winding strip such that the L-shaped cover overlaps each of the first and second insulation covers and the first and second leg portions.
According to further embodiments of the invention, a winding assembly for use in a transformer includes a foil strip. The foil strip includes a winding portion and a breakout portion integral with the winding portion and joined to the winding portion along a fold. A first insulation cover covers a portion of the winding portion. A second insulation cover covers a portion of the breakout portion. A supplemental insulation cover covers each of the winding portion and the breakout portion.
According to further embodiments of the invention, a winding assembly for use in a transformer includes a foil winding strip having a lengthwise axis and a first insulation cover covering a portion of the winding strip. An exposed portion of the winding strip extends beyond the first insulation cover along the lengthwise axis of the winding strip. The winding assembly further includes a foil breakout strip having a lengthwise axis. A second insulation cover covers a portion of the breakout strip. An exposed portion of the breakout strip extends beyond the second insulation cover along the lengthwise axis of the breakout strip. The breakout strip exposed portion engages the winding strip exposed portion and the second insulation cover overlaps the winding strip.
According to embodiments of the invention, a winding assembly for use in an electrical transformer includes a foil winding portion and a foil breakout portion adjoining the winding portion. The breakout portion overlaps and extends at an angle with respect to the winding portion. A supplemental insulation cover is wrapped about and between the winding portion and the breakout portion. The supplemental insulation cover includes a first panel covering the breakout portion, a second panel covering a rear surface of the winding portion, and a third panel disposed between the winding portion and the breakout portion.
According to further embodiments of the invention, a winding assembly for use in a transformer includes a foil winding strip and a foil breakout strip. The foil winding strip has a lengthwise axis. A first insulation cover covers a portion of the winding strip. An exposed portion of the winding strip extends beyond the first insulation cover along the lengthwise axis of the winding strip. The foil breakout strip has a lengthwise axis. A second insulation cover covers a portion of the breakout strip. An exposed portion of the breakout strip extends beyond the second insulation cover along the lengthwise axis of the breakout strip. The breakout strip exposed portion engages the winding strip exposed portion and the breakout strip and the winding strip form an inner corner therebetween. A bellows cover covers the breakout strip and the winding strip. The bellows cover includes a first leg portion overlapping the first insulation cover, a second leg portion overlapping the second insulation cover, and a bellows section extending across the inner corner and joining the first and second leg portions. The winding assembly may further include an L-shaped cover covering the breakout strip and the winding strip, the L-shaped cover overlapping each of the first and second insulation covers and the first and second leg portions.